1. Field of the Invention
The invention is directed to a filter such as a comb line or an interdigital line filter which uses flat spiral resonators.
2. Description of the Prior Art
The article entitled "Band-Pass and Band-Stop Microwave Filters Using .lambda./4 Circular Cylindrical Real Resonators" appearing in the Fujitsu Scientific Technical Journal, Vol. 4, No. 3, Pages 29 through 52 by Juhio Ito and Takeshi Meguro describes line filters. For mobile radio directional links and satellite links, transmission/reception diplexers and IF band-pass filters having high selectivity and low losses are required.
In addition to the demand for high resonator quality, small volme, low weight and cheap manufacturing costs for mass production are required in mobile radio such as, for example, for automobile telephones.
Until this time, filters were constructed using helix resonators as described in the publication by B. K. Dube, "The Design of Filters Using Helical Resonators in VHF-Band" appearing in the Journal of Institute of Electronics Telecom. Engineers, Vol. 22, No. 2 1976, pages 77 through 79. Alternatively, such filters were constructed using resonators in the form of metal rods for example, as comb or interdigital filters as disclosed in the Journal of Institute Electronics Telecom Engineers referenced above wherein ceramics such as described in U.S. Pat. No. 4,431,977 is used as the dielectric in addition to air thus reducing the length of the metal rod and the volume by the factor of .sqroot./.epsilon. where .epsilon. is the dielectric constant of the ceramic.
Filters are also known in which planar spiral coils on a ceramic substrate are combined with discrete capacitors to form series circuits and are interconnected to form a band-pass filter. Neither high resonator quality nor low cost manufacturing are achieved with such technique.
Helix-shaped filters have relatively high manufacturing costs and many individual parts. The filters constructed with metal rods which use air dielectric are bulky and those having a ceramic dielectric are relatively heavy which is undesirable particularly in mobile devices.